Change of regime of decay of elastic precursor wave in BCC metals

Our studies of decay of elastic precursor wave with propagation distance in five BCC metals, namely V, Ta, Fe, Nb, and Mo show that at propagation distances of about h * = 1 mm the regime of the decay is changed. At propagation distances smaller than h * the decay is fast and the spatial variation of the elastic wave amplitude σHEL is described by the power function σHEL =σ0(h /h0)-α with α ranged between 0.3 and 0.7 for different metals at different temperatures. Beyond the distance h * the decay is much slower and is characterized by much lower values of α, of about 0.1 or less. The stresses τ* at which the transition occurs at room temperature is close to the Peierls stresses τP of the studied metals. This allows us to conclude that the change of the decay regime at τ* is caused by the change of the mode of the dislocations motion from the over-barrier glide controlled by the phonon viscous drag above τ* to that controlled by thermally activated generation/motion of the dislocation double-kinks below τ*. The decline of τ* with temperature (~ 50% over 1000-K interval) agrees with the growing with temperature support of dislocation motion by thermal fluctuation.